A subject matter of the present invention is a reforming tube comprising a means which makes it possible to prevent, in its cavity, the corrosion phenomenon known as metal dusting. Metal dusting is understood to mean a catastrophic disintegration of the metals (Fe, Ni, Co) observed in carburizing atmospheres of chemical and petrochemical processes.
This rapid and local corrosion can result in problems within the reforming process, in particular safety problems.
Today, the mechanisms of this corrosion are not well understood and it appears that numerous parameters have an influence: the chemical composition of the alloy, the size of the grains, the residual stresses, the finishing of the surface, the pressure of the process, the composition of the atmosphere, and the like, and it is difficult to evaluate the intrinsic contribution of each parameter and their interactions.
Over many years, researchers have developed several barriers for preventing this corrosion phenomenon known as metal dusting.
Among the solutions developed, the most important is the aluminizing of the alloy. This consists in reacting aluminum vapor with the alloy in order to form an aluminum-rich exterior layer. The chemical composition of the coating can vary according to the alloy. For example, if an alloy based on Cr—Ni is considered, the exterior layer will contain Al, Ni and Cr.
More recently, researchers have developed a novel coating formulation with the objective of preventing the formation of carbon from the atmosphere. It is known today, for example, that nickel can catalyze the formation of carbon from a cementation atmosphere and that this is the first stage of the mechanism of the metal dusting phenomenon. By using an exterior layer which does not catalyze the formation of carbon, the formation of carbon and thus the corrosion phenomenon known as metal dusting are thus prevented. Tin-comprising coatings have been developed and appear to perform well.
Composite tubes are another solution for reducing corrosion. This solution is based on the assembling of two materials having dissimilar properties. Usually, one material will exhibit a high resistance to corrosion under the conditions of the process while the other will exhibit elevated mechanical properties. The two materials, usually alloys, are welded together using different manufacturing processes. This results in two layers of materials having different thicknesses according to the specifications of the process. The main difficulty is to produce complex shapes and the problem of thermal expansion, resulting in stresses which might increase the rate of corrosion or reduce the mechanical strength.
Taking this as the starting point, the first limitation of the existing solutions with regard to the corrosion phenomenon known as metal dusting is the size of the parts to be protected. For example, if the tubes used for the reforming of methane are considered, these tubes are approximately 13 m in length and it is difficult to find firms which have the appropriate equipment for producing a homogeneous (thickness and chemical composition) coating. In addition, it is difficult to produce the coating on small sections of tube, before welding them together in order to obtain the final length, due to the low welding capability of coatings made of aluminum on small sections.
The configuration of the parts to be protected can also reduce the choice in terms of coating techniques and chemical composition of the coating.
Another of the problems encountered is the difference in expansion behavior between the alloy of the reforming tube and the coating, which might result in cracks.
Consequently, it is a matter of finding a reforming tube which exhibits a new improved means making it possible to prevent, in its cavity, the corrosion phenomenon known as metal dusting.